A compressor flow map, e.g., a plot of pressure ratio versus mass air flow, can help characterize performance of a compressor. In a flow map, pressure ratio is typically defined as the air pressure at the compressor outlet divided by the air pressure at the compressor inlet. Mass air flow may be converted to a volumetric air flow through knowledge of air density or air pressure and air temperature. Compression causes friction between air molecules and hence frictional heating. Thus, air at a compressor outlet generally has a considerably higher temperature than air at a compressor inlet. Intercoolers act to remove heat from compressed air before the compressed air reaches one or more combustion chambers.
A typical compressor flow map usually indicates compressor efficiency. Compressor efficiency depends on various factors, including pressure, pressure ratio, temperature, temperature increase, compressor wheel rotational speed, etc. In general, a compressor should be operated at a high efficiency or at least within certain efficiency bounds. One operational bound is commonly referred to as a surge limit while another operational bound is commonly referred to as a choke area. Compressor efficiency drops significantly as conditions approach the surge limit or the choke area.
Choke area results from limitations associated with compressor wheel rotational speed and the speed of sound in air. In general, compressor efficiency falls rapidly as compressor wheel blade tips exceed the speed of sound in air. Thus, a choke area limit typically approximates a maximum mass air flow regardless of compressor efficiency or compressor pressure ratio.
A surge limit exists for most compressor wheel rotational speeds and defines an area on a compressor flow map wherein a low mass air flow and a high pressure ratio cannot be achieved. In other words, a surge limit represents a minimum mass air flow that can be maintained at a given compressor wheel rotational speed and a given pressure difference between the compressor inlet and outlet. In addition, compressor operation is typically unstable in this area. Surge may occur upon a build-up of back pressure at the compressor outlet, which can act to reduce mass air flow through the compressor. At worst, relief of back pressure through the compressor can cause a negative mass air flow, which has a high probability of stalling the compressor wheel. Some compressor systems use a bypass valve to help relieve such back pressure and thereby avoid any significant reduction of mass air flow through the compressor. Surge prevention can also reduce wear on a compressor and related parts.
Overall, surge of centrifugal compressors limits the useful operating range. Previous attempts to reduce surge limits for compressors have met with difficulties at low compressor wheel rotational speeds. For example, various previous attempts used a port between the compressor outlet and the compressor inlet to re-circulate some of the air mass when a build-up of back pressure occurred. However, such a port significantly reduced compressor efficiency.
Various previous attempts have also used a ported compressor wheel shroud (e.g., a shroud having a port) to reduce surge flow at higher compressor wheel rotational speeds, but at lower compressor wheel rotational speeds, the pressure difference between the port and the compressor inlet is typically very small and hence the port becomes fairly ineffective.
While previous attempts at re-circulation have proven less than ideal, especially at low compressor wheel rotational speeds, re-circulation remains as a viable means to improve compressor operation. In particular, as presented herein, various exemplary ports, systems and/or methods, rely on a specialized port to improve compressor operation at lower compressor wheel rotational speeds and/or at other operating conditions.